To improve the range of a commercial FM transmitter station in hilly regions, it is known to use several transmitting stations operating at the same frequency which are located at critical locations. However, the operation of a network of transmitters each using a separate conventional pilot oscillator is almost impossible since the frequency drift between the oscillators is so great that it is impossible to keep the transmitters operating at the same frequency more than a few seconds. High stability oscillators must absolutely be used. However, these oscillators are very expensive and, more importantly, after having been adjusted in frequency one with respect to the other, start immediately to drift apart slightly in frequency so that, after a few months of operation, frequency netting must be done by a competent technician. Thus, although high stability oscillators are excellent on a short term basis, they are still deficient over long periods of time.
The result of such frequency drift is a low frequency beat between two neighboring transmitting stations which is sensed by a receiver located within the broadcasting range of the two transmitting stations. The signal received is hampered by such low frequency beat and, as the drift increases, the message becomes unintelligible. Then, frequency netting of the transmitting stations must be done. This operation is expensive and add to the maintenance of the network.